This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-53648 filed on Feb. 28, 2001.
The present invention relates to a discharge lamp unit with noise shields and a noise control method for a discharge lamp unit.
One proposed arrangement for noise control in a discharge lamp unit is shown in FIG. 7. A discharge lamp 200 includes a discharge lamp body 201 and a connector 202. The connector 202 is electrically connected with a discharge lamp control circuit 210 which regulates a voltage applied to the discharge lamp 200. A filter circuit 211 is connected between the control circuit 210 and a power supply 230 to reduce noise radiation and conduction from the discharge lamp 200 or the control circuit 210 to the power supply 230.
In this arrangement, noise produced during an operation of driving the discharge lamp 200 with a rectangular wave is conducted to the control circuit 210. When a DC/DC converter in the control circuit 210 is switched to step up a voltage to the level required for a start or steady lighting of the discharge lamp 200, switching noise is produced. In the case of the discharge lamp 200 being used for a headlight of a vehicle, noise in power supply lines may result in radio noise. To avoid this, a filter circuit 211 is connected between the control circuit 210 and the power supply 230. Moreover, the control circuit 210 is shielded with a shield case 220 to reduce noise radiation therefrom.
Although the noise from the control circuit 210 can be reduced by the shield case 220, it may still be radiated outside the shield case 220 via wires 240. To counter this problem, another arrangement, as shown in FIG. 8, is proposed. In this proposed arrangement, the control circuit 210, the filter circuit 211, and the wires 240 are shielded by a shield case 221. However, this does not provide sufficient shielding since the noise may be conducted to the power supply 230 or radiated via wires 241.
The present invention therefore has an objective to provide a discharge lamp unit with noise shields which enable superior control of noise radiation and noise conduction from a discharge lamp or a discharge lamp control circuit to a filter circuit and a power supply.
Another objective of the present invention is to provide a superior noise control method for a discharge lamp control unit.
According to the present invention, the second shield case is installed to exclusively shield the control circuit in addition to the first shield case shielding the discharge lamp control circuit and the filter circuit. One of the wires connecting the filter circuit to the power supply, referred to as a ground wire and the first shield case are electrically connected near the shield case opening through which the ground wire passes. As a result, noise is reduced even if noise produced by the discharge lamp or the control circuit is conducted outside the first shield case via the wires.